Photochemistry of 4-Chloroaniline in Solution. Formation and Kinetic Properties of a New Carbene, 4-Iminocyclohexa-2,5-dienylidene

Abstract

The photochemistry of 4-chloroaniline and related 4-halogenoanilines was investigated in solution by means of nanosecond transient absorption spectroscopy and steady-state photoproduct analysis. The main emphasis was the study of the mechanism of photoinduced reactions in aqueous solutions, motivated by the presence of 4-chloroaniline in the environment, following biodegradation of chlorophenylurea-based herbicides. Additional measurements were carried out in some organic solvents (aliphatic alcohols, acetonitrile, and n-hexane) in order to clarify the mechanism. The first transient detectable at nanosecond time resolution in aqueous solution was assigned to a carbene, 4-iminocyclohexa-2,5-dienylidene, produced by photoinduced heterolytic dehalogenation. The spectral and kinetic properties of this previously unknown species, as well as the products of its reactions, were studied in detail; it was found to react with O 2 , giving an iminoquinone O-oxide, to abstract an H atom from aliphatic alcohols, giving an anilino radical cation, to add bromide, giving 4-bromoaniline quantitatively, and to react with H 2 O quite slowly. This behavior, characteristic of a carbene with a triplet ground state, is fully analogous to that found in the well-studied photochemistry of 4-halogenophenols. There are a few characteristic differences in this system, however, most conspicuous being that the amino (or imino) group can accept a positive charge. For this reason, contrary to 4-chlorophenol, photolysis of 4-chloroaniline also produced the carbene in nonprotic polar solvents, such as acetonitrile. Furthermore, contrary to 4-chlorophenol and 4-chloroanisole, the reaction mechanisms of 4-chloro-N,Ndimethylaniline and 4-chloroaniline are the same. Finally, the product distribution from 4-chloraniline in the presence of O 2 was found to be different from that of 4-chlorophenol, and this was traced to the fact that the iminoquinone oxide is able to add to 4-chloroaniline.

Direct phototransformation of aromatic pesticides in aqueous solution

“…This photocontraction leads to cyclopenta-1,3-diene-1carbonitrile [8]. With 4-halogenoanilines the first step is the formation of a cationic carbene [9]. According to the conditions it leads to the formation of 4-aminophenol, aniline, diphenylamine derivatives or substituted biphenyls.…”

Photochlorination of aniline in Fe3+-containing saline water under simulated solar light irradiation

“…[13,14] It was reported that anilino radical cations showed absorptions at 407 and 423 nm, and chloroanilino radical cations showed absorption at 447 nm in the transient absorption spectra of LFP. [15,16] The absorptions observed at ,406, ,424 and ,448 nm were most likely from the anilino radical cations and the CA radical cations as shown in Fig. 3.…”

Photochemistry of Oxazolidinone Antibacterial Drugs†

“…Insertion of a fluorine decreases both the fluorescence and the phosphorescence of these compounds, thus no indication about the multiplicity of the reacting states comes from this fact. However, with simple anilines, including some fluorinated anilines (34), intersystem crossing in solution is quite efficient ( Φ ISC ≈ 0.7) and thus it seems possible that the photoreactions considered, with quantum yield up to 0.33, originates from the triplet ( 3 20 in Scheme 3). The type of chemistry observed then supports this idea, as shown below.…”